A CTA Purple Line train passes over the viaduct at Greenleaf Street

A CTA Purple Line train passes over the viaduct at Greenleaf Street, one of three viaducts in Evanston slated for replacement by the transit agency. (Jonathan Bullington/TribLocal)

A CTA Purple Line train passes over the viaduct at Greenleaf Street, one of three viaducts in Evanston slated for replacement by the transit agency. (Jonathan Bullington/TribLocal) ((Jonathan Bullington/TribLocal))

A CTA Purple Line train passes over the viaduct at Greenleaf Street, one of three viaducts in Evanston slated for replacement by the transit agency. (Jonathan Bullington/TribLocal)